Abstract

This paper discusses the implications of drying and flooding effects on nutrient release from sediments in wetlands. It discusses the role of sediment properties, phosphorus and nitrogen transformations, carbon cycling, drawdown and macrophyte control, and modelling. Finally, comments on management issues are made. In summary: Drying and refilling cause substantial changes in water quality. The effect depends on 1. Sediment properties (sediment composition; nutrient and organic content); 2. type of drawdown (gravity or evaporative); 3. severity of drying (proportion of drying area, rate of drawdown, degree of dewatering, temperature and time of sediment drying and weathering); 4. conditions of refilling (origin of water, degree of sediment disturbance). Sediment properties are the key to physical, chemical and biological changes during drying and reflooding, as they affect nutrient transformations and exchange between sediment and water. Changes may include particle aggregation and sediment consolidation, phosphorus adsorption and desorption; organic degradation, nitrogen transformation, and stimulation of microbial processes. Phosphorus transformations in sediments during drawdown depend on many factors. There is a higher probability of stimulating phosphorus release than of reducing phosphorus concentration during the reflooding of dried sediments, particularly in organic-rich wetlands. Stimulation of phosphorus release and algal production is most likely to result from an evaporative drawdown event. Concentrations of inorganic nitrogen (ammonia and nitrate) often increase during or after drawdown, although the effect may be short-term. Such release has the potential to stimulate N-limited algal blooms. Drawdown has been used effectively in controlling macrophytes. The levels of sediment dewatering, freezing or hot climate conditions generally determine the levels of macrophyte reduction. However, control of macrophytes is often species specific. The elimination of macrophytes may increase the probability of algal blooms.